1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gas. More particularly, this invention relates to a catalyst for removing hydrocarbons (hereinafter referred to simply as "HC"), carbon monoxide (hereinafter referred to simply as "CO"), and nitrogen oxides (hereinafter referred to simply as "NOx"), which are noxious components contained in the exhaust gas. To be still more specific, this invention relates to a catalyst for purifying the exhaust gas, which enables HC, CO, and NOx contained in the exhaust gas to be simultaneously removed efficiently at a low temperature while an internal combustion engine is being operated with the air/fuel (A/F) ratio controlled near the stoichiometric equivalent point.
2. Description of Prior Art
Heretofore, numerous catalyst have been proposed for purifying the exhaust gas emanating from the internal combustion engine such as of an automobile. At present, three way catalysts capable of simultaneously removing CO, HC and NOx are predominant over other catalysts.
Any of the aforementioned catalysts for purifying the exhaust gas is composed mainly of a carrier and a catalytically active seed. As the catalytically active species, such precious metals as platinum (Pt), palladium (Pd), and rhodium (Rh) are chiefly used. For the purpose of accelerating and stabilizing the catalytic activity of such precious metals, compounds selected from the compounds of such rare earth metals as cerium, lanthanum, and praseodymium, the compounds of such Group VIII elements as iron, cobalt, and nickel, the compounds of such alkali metals as sodium, potassium, and cesium, and the compounds of such alkaline earth metals as barium and calcium are generally used. Among other compounds enumerated above, the oxides of cerium prove particularly useful.
It is said that the oxides of cerium generally have an effect of oxygen storage, an effect of promoting the water gas shift reaction, an effect of enhancing the dispersibility of a precious metal, and an effect of improving thermal stability. How these oxides of cerium are to be effectively utilized constitutes itself a very important problem.
In recent year, since the temperature of the exhaust gas from the automobile is tending downwardly because of the efforts devoted to economizing the fuel consumed in the automobile, a need has developed for three way catalyst which excels in ability to effect removal of CO, HC and NOx from the exhaust gas at low temperatures. As means of depositing a given precious metal on the conventional rhodium-containing honeycomb catalyst used for purifying the exhaust gas, there have been proposed a method which effects the deposition by coating three way honeycomb carrier with active alumina and then impregnating the coated carrier with the precious metal solution and a method which effects the deposition by impregnating active alumina pellets with the precious metal solution. In either of the methods, the rhodium is deposited by impregnated simultaneously with at least one precious metal (a) selected from the group consisting of platinum and palladium. Thus, the rhodium which is required to be used in a limited minute amount cannot be dispersed and deposited in the optimum state. As the result, the acquisition of satisfactory catalytic activity has been unsuccessful.
In the specification of U.S. Pat. No. 4,283,308 is disclosed a method which causes a precious metal (a) and rhodium to be deposited independently of each other on alumina pellets. By this method, the precious metal is distributed solely on the surface of alumina pellets about 3 mm in diameter.
Methods which effect the deposition of a precious metal (a) on a carrier in the form of pellets by using a solution of the precious metal (a) which additionally incorporates a polybasic carboxylic acid are disclosed in the specification of U.S. Pat. No. 3,259,454, the specification of Japanese Patent Publication No. SHO 59(1984)-33,019, and the specification of Japanese Patent Laid-open No. SHO 51(1976)-21,592. These methods invariably cause the precious metal (a) to be distributed within alumina pellets 2 to 4 mm in diameter. The specification of Japanese Patent Laid-open SHO 59(1984)-154,139 discloses a method which effects simultaneous deposition of Pt, Pd, and Rh on a monolithic honeycomb carrier by immersing the carrier in a solution of the precious metals which additionally incorporates therein an organic acid. Similarly to the method using a carrier in the form of pellets, this method causes the precious metals to be distributed within the active coating applied on the honeycomb carrier. These conventional methods invariably consist in forming an ununiform concentration distribution of a given precious metal (a) within pellets of the carrier or within the active coating deposited on the honeycomb carrier.
An object of this invention, therefore, is to improve the function of cerium oxide which has been finding growing recognition as an essential component for the catalyst used for purifying the exhaust gas.
Another object of this invention is to provide a catalyst for purifying the exhaust gas which has the CO conversion improved and the A/F region in which CO, HC, and NOx are simultaneously removed in high levels, i.e. the window width, broadened by enhancing the effect of cerium oxide in oxygen storage and in promoting the water gas shift reaction.
Yet another object of this invention is to provide a catalyst for purifying the exhaust gas which, by improving the method for the deposition of a precious metal on active alumina, is enabled to exhibit an excellent ability to remove CO, HC, and Nox at low temperatures.
A further object of this invention is to provide a method for manufacture of the catalyst for purifying the exhaust gas which excels in low-temperature activity.